Oxides of nitrogen (NOx) represent a significant pollutant that is regulated across the globe. The mitigation of NOx generated by internal combustion engines is a primary function of catalytic aftertreatment systems. Different engine types and different fuels produce exhaust having different NOx levels and different constituent component concentrations contributing to NOx. To evaluate the performance of such catalytic aftertreatment systems, it is necessary to control the amount of NOx present in the exhaust sample. However, since NOx includes both nitrogen oxide (NO) and nitrogen dioxide (NO2), simply controlling the overall NOx concentration is inadequate to evaluate the performance of a catalyst or catalytic aftertreatment system. To properly develop emission control systems, it is necessary to specifically control the appropriate NO/NO2 concentration ratio in the exhaust such that the test exhaust composition produced by a reactor represents the true composition of the actual exhaust produced by an internal combustion engine.